In general, polyesters such as polyethylene terephthalate are excellent in mechanical properties and chemical properties, and are widely used as formed products such as films and fibers. When the polyesters are processed and used as formed products, their slipping property and abrasion resistance greatly affect the workability in their production and various applications. For example, if a polyester tape insufficient in such properties is used as a base film for producing a magnetic tape, the friction between the coating roll and the film during magnetic tape production becomes large, thereby wrinkling or scratching the film. Moreover, the film is liable to be abraded to generate powder, and when the film is coated with a magnetic layer, it may have portions devoid of the magnetic layer, and as a result, missing of magnetic recording (drop-out) is likely to occur. Furthermore, as a recently identified disadvantage, when the film is wound after completion of calendering in the step of magnetic layer coating, the magnetic face is rubbed by the non-magnetic face, to be flawed, thereby causing quality deterioration such as drop-out in use as a product.
For improving the slipping property of a film, many proposals have been made for letting polyesters contain inorganic particles such as titanium dioxide, calcium carbonate and silicon dioxide. However, these inorganics particles which are low in affinity to polyesters easily fall off, producing scrapings, degrading in slipperiness or suffering from flaws on the surface, when an external force acts during use, for example, as a film, due to contact with rolls during calendering in the step of magnetic layer coating or during film running or due to inter-layer contact of the film during winding. If such scrapings are produced in large amounts or the surface is flawed, it would lead to imperfect formation of the magnetic layer during the coating process and the film may suffer from drop-outs during use. Furthermore, if the calender roll used in the step of magnetic layer coating is soiled, the workability in the production of magnetic recording film is remarkably lowered. Thus it has been hoped that the properties of these magnetic layers may be improved so that they do not suffer from flaws and abrasion due to low affinity to polyesters.
Methods proposed for improving the affinity between the particles and polyesters include the surface treatment of inorganic particles, the use of organic particles, etc. Conventional methods regarding surface treatment of inorganic particles include the surface treatment by polyacrylic-acid-based polymers proposed in Japanese Patent Laid-Open (Kokai) No. 63-128031, the surface treatment by phosphorus-containing compounds proposed in Japanese Patent Laid-Open (Kokai) Nos. 62-235353 and 63-234039, the surface treatment by coupling agents proposed in Japanese Patent Laid-Open (Kokai) Nos. 62-223239 and 63-312345, the surface treatment by silane compounds proposed in Japanese Patent Laid-Open (Kokai) No. 63-304038 and the surface treatment by grafting using a glycol proposed in Japanese Patent Laid-Open (Kokai) No. 88-280763. However, even these methods cannot achieve sufficient abrasion resistance since it is difficult to obtain satisfactory affinity because the particles used are inorganic.
For organic particles, various kinds are proposed, for example, in Japanese Patent Laid-Open (Kokai) No. 49-117550 (polyethylene terephthalate particles and poly-1,4-bis(hydroxymethyl)-cyclohexane terephthalate particles), Japanese Patent Laid-Open (Kokai) No. 50-14748 (polyethylene naphthalate resin), Japanese Patent Laid-Open (Kokai) No. 54-132652 (polyphenyl ester resin), etc. However, since these particles are of linear polymers without crosslinking structure, they are insufficient in heat resistance, and the thermal history of high temperatures inflicted during the production of polyesters and films makes the particles poorly stable, thereby disadvantageously not allowing the films to be sufficiently resistant against abrasion. Furthermore, organic particles with crosslinked structure are proposed, for example, in Japanese Patent Laid-Open (Kokai) No. 59-217755 (crosslinked polymer particles sharp in grain size distribution), Japanese Patent Laid-Open (Kokai) No. 61-174254 (crosslinked polymer particles capable of reacting with polyesters) and Japanese Patent Laid-Open (Kokai) No. 2-189359 (vinyl compound/divinylbenzene copolymer particles, dicarboxylic acid/acrylate copolymer particles, etc.), etc. However, a problem of these particles is that they are not of polyesters, even though they are relatively high in heat resistance. Since ester bonds do not exist at the main chain of the polymer forming the particles, the polymer cannot exhibit its affinity with the polyester used as the matrix, and the film containing them cannot have high abrasion resistance when it runs at a high speed.
The inventors studied intensively, and as a result, found that particles made of a crosslinked polymer with ester bond at its main chain are very high in affinity with polyesters, and that films containing them are remarkably higher in abrasion resistance and magnetic face flaw preventability when they are run at high speeds. The object of the present invention is to overcome the above mentioned disadvantages of the prior art, and to present polyester composition excellent in abrasion resistance and magnetic face flaw preventability when the films formed from them are run at high speeds.